In electric-discharge machining (EDM), an electrode is spacedly juxtaposed with a workpiece across a gap through which a dielectric liquid is forced while a power supply capable of providing electrical pulses sufficient to effect dielectric breakdown across the gap is connected in circuit with the electrode and the workpiece. With each discharge, a portion of the workpiece is eroded and the detritus is carried away by the dielectric liquid and a nonconductive or low-conductivity state is re-established in the gap.
In early systems of this type, the machining-pulse source was a capacitor connected across the gap and charged by a direct-current source connectable in parallel therewith, the gap breaking down and sustaining the machining discharge when the potential across the electrodes reached the breakdown potential of this gap. These systems had the disadvantages that the discharge-pulse energy varied from discharge to discharge, that short-circuiting prevented the buildup of the capacitor charge while conditions close to short-circuiting prevented significant energy storage in the capacitor, and that significant opening of the gap prevented any discharge whatsoever.
To overcome these disadvantages, switch-pulse systems have been employed wherein an electric-discharge machining current source is connected in circuit with the gap by ON-OFF switch means of an electronic, mechanical or electromechanical type and the switch means is altered in state between its conductive and nonconductive conditions to apply a substantially rectangular waveform discharge pulse across the gap. In prior systems of this gendre, the switch means was controlled by a multivibrator or other ON-OFF signal generator and it was quickly found that the parameters of the electrical pulse had to be established most carefully for efficient machining, especially where electrode wear is to be limited ("no wear") and energy loss is to be a minimum.
Various disturbances arise in electric-discharge machining and, since they have been discussed in some detail in my earlier applications mentioned above, it is necessary only to refer to several of them to place the present developments in EDM pulse control in the proper perspective. It is desired to have the pulses or discharges be of equal energy from pulse to pulse (isoenergy pulses). However, detritus does not always clear uniformly from the gap and residual ionization may make high energy pulses premature from time to time. The tool electrode may approach the workpiece too closely so that a particular discharge energy may be excessive or the gap may be short-circuited in which case the discharge will merely exacerbate the problem. Finally, arcing (continuous discharge) may occur if insufficient gap recovery time is permitted or some other gap parameter is altered. Consequently, with multivibrator or like control of ON-OFF time for the discharge pulse, it was also proposed to vary the pulse duration, the interval between the successive pulses and the pulse height on a per-pulse basis. It was also found, as discussed in the aforementioned applications, that self-adpative control on a per-pulse basis was desirable, in which case means was provided to detect a gap condition indicative over the status of the gap during the next discharge period for controlling the subsequently developed discharge is to amplitude, duration or interval.